Method for achieving a thin film of solid material and applications of this method

ABSTRACT

A method for making a thin film of solid material, including bombarding one face of a substrate of the solid material with at least one of rare gas ions and hydrogen gas ions so as to create a layer of microcavities seperating the substrate into two regions at a depth neighboring the average ion penetration depth, and heating the layer of microcavities to a temperature sufficient to bring about a separation between the two regions of the substrate. The solid material includes one of a dielectric material, a conducting material, a semi-insulating material, and an unorganized semiconducting material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. §371 ofPCT/FR97/00842 filed May 13, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns a method for achieving a thin film ofsolid material, this material possibly being a dielectric, a conductoror a semi-insulator. It may be crystalline or noncrystalline. It mayconsist of an amorphic or polycrystalline semiconductor whosecrystallographic planes are oriented in any direction whatsoever. Thismaterial may have ferroelectric, piezoelectric, magnetic, electro-opticproperties, etc.

A particularly interesting application of the method according to theinvention concerns the achievement of ferroelectric capacitor memories.

2. Description of the Background

Various methods are known to achieve thin films of solid material. Thesemethods depend on the nature of the material and the thickness of thefilm desired. For instance, it is possible to deposit thin films of asolid material on the surface of a piece by projection, spraying,electroplating, etc. A thin film may also be obtained by thinning aplate of the material desired by mechanical-chemical or chemicalabrasion, the thin film obtained then being bonded or fixed on a pieceserving as a support.

In general, a thin film is fixed on the surface of a piece so as tomodify the properties of the piece superficially.

In the semiconductor field, it is also sometimes necessary to achievethin semiconducting films, for example to manufacture so-called“silicon-on-insulator” substrates. Various techniques to achieve thinsemiconducting films have been developed. One of the most recenttechniques is based on the fact that the implantation of rare gas orhydrogen ions in a semiconducting material induces the formation ofembrittled areas at a depth neighbouring the average ion penetrationdepth. Document FR-A-2 681 472 discloses a method which makes use ofthis property to obtain a thin film of semiconducting material.According to this method, a plate of the semiconducting material desiredcomprising a flat face is subjected to the following steps:

a first implantation step wherein the flat face of the plate isbombarded with ions, thus creating, within the plate body and at a depthneighbouring the ion penetration depth, a “gaseous microblister” layerseparating the plate into a lower region making up the mass of thesubstrate and an upper region making up the thin film, the ions beingchosen among rare gas or hydrogen gas ions;

a second step wherein the flat face of the plate is placed in closecontact with a support made up of at least a layer of rigid material.This close contact being achieved, for example, using an adhesivesubstance or through the effect of a prior preparation of the surfacesand possibly a heat and/or electrostatic treatment to favour the atomicbonds between the support and the plate;

a third heat treatment step wherein the plate-support assembly is heatedto a temperature higher than the temperature at which the implantationwas performed and sufficient to create a separation between the thinfilm and the mass of the substrate. This temperature is higher than orequal to approximately 400° C. for silicon.

This implantation is suited to create a layer of gaseous microblisterswhich will result in a rupture area at the end of the heat treatment.The layer of microblisters thus created in the plate body, at a depthneighbouring the average ion penetration depth, delimits two regions inthe plate body separated by this layer: a region intended to make up thethin film and a region forming the rest of the substrate. During thethird step, the heat treatment is performed at a temperature sufficientto create the rupture area and the separation between the two regionsthrough a crystalline rearrangement effect in the semiconductingmaterial, for example through a microcavity growth effect and/or amicroblister pressure effect.

Depending on the implantation conditions, following the implantation ofa gas such as hydrogen for example, cavities or microblisters may or maynot be observed with a transmission electron microscope. In the case ofsilicon, microcavities of sizes ranging from a few nm to a few hundrednm may be present. As a result, these cavities are only observable inthe heat treatment step, particularly when the implantation temperatureis low, and nucleation is therefore performed during this step so as toobtain the rupture between the thin film and the rest of the substrateat the end of the heat treatment.

Until now, it was believed that the method disclosed in document FR-A-2681 472 could only be applied to achieve a thin film using a substrateof semiconducting material. This document provides the followingexplanation to the various phenomena known from experience. First ofall, the first ion implantation step is carried out by exposing a flatface of a plate of semiconducting material to a beam of ions, the planeof this flat face being either substantially parallel to a maincrystallographic plane in the case where the semiconducting material isperfectly monocrystalline, or slightly inclined with respect to a maincrystallographic plane with the same index for all grains in the casewhere the material is polycrystalline. This leads to the creation, inthe plate body at a depth neighbouring the average ion penetrationdepth, of a layer of “gaseous microblisters” corresponding toembrittlement areas and delimiting, in the plate body, two regionsseparated by this layer: a region intended to make up the thin film anda region forming the rest of the substrate. The term “gaseousmicroblisters” refers to any cavity or microcavity generated by theimplantation of hydrogen gas or rare gas ions in the material. Thecavities may have either a very flat shape, i.e. with a small height,for example in the order of a few atomic gaps, or a substantiallyspherical shape or any other shape different from the two previousshapes. These cavities may or may not contain a free gaseous phaseand/or gas atoms derived from implanted ions fixed to atoms of thematerial forming the walls of the cavities. These cavities are generallyreferred to as “microblisters”, “platelets or even ” “bubbles”. Duringthe third step, the heat treatment is performed at a temperaturesufficient (for the duration of the treatment applied) to create theseparation between the two regions. The time-temperature pair of theheat treatment depends of the dose of implanted ions.

The method described in document FR-A-2 681 472 concerns the achievementof a thin film using a substrate of semiconducting material having acrystalline structure. The development of the various steps of themethod has been explained as resulting from the interaction between theimplanted ions and the crystalline mesh of the semiconducting material.

However, the inventors of the present invention were surprised todiscover that this method could be applied to all types of solidmaterials, crystalline or noncrystalline. This method may be applied todielectric, conducting, semi-insulating materials, as well as toamorphic semiconducting materials and even polycrystallinesemiconductors whose grains do not have main crystallographic planessubstantially parallel to the flat face of the plate. The latter, alongwith amorphic semiconductors, will be referred to as unorganizedsemiconductors hereinafter. Furthermore, this method does notsignificantly modify the properties of the material it is applied to.

The inventors of the present invention were surprised to discover thatthe implantation of hydrogen gas or rare gas ions may also bring aboutthe formation of microcavities in solid materials other than acrystalline semiconducting material, and that a subsequent heattreatment may bring about the separation, at the microcavities, of themass of the material into two parts. Indeed, the heat treatment induces,regardless of the type of solid material, the microcavities to coalesce,which leads to an embrittlement of the structure at the layer ofmicrocavities. This embrittlement enables the separation of the materialunder the effect of internal stresses and/or pressure within themicrocavities, this separation being natural or assisted by externalstresses.

The term layer of microcavities refers to an area containingmicrocavities possibly located at different depths and adjacent or notadjacent to one another.

SUMMARY OF THE INVENTION

The object of the invention is therefore a method for achieving a thinfilm of solid material, crystalline or noncrystalline, chosen among adielectric material, a conducting material, a semiinsulating material,an unorganized semiconducting material, characterised in that asubstrate of said solid material is subjected to the following steps:

an ion implantation step during which one face of the substrate isbombarded with ions chosen among rare gas and hydrogen gas ions so as tocreate, in the body of the substrate at a depth neighbouring the averageion penetration depth, a layer of microcavities separating the substrateinto two regions,

a heat treatment step intended to heat the layer of microcavities to atemperature sufficient to bring about a separation between the tworegions of the substrate, either naturally or by means of an appliedstress.

It is possible to include, between the ion implantation step and theheat treatment step, a step wherein said face of the substrate is fixedon a support. This step may be necessary in the case where the thinlayer is not sufficiently rigid on its own. It may be desired since thethin layer is generally intended to be placed on a support. In thiscase, the support must be able to withstand the final heat treatmentstep.

Said face of the substrate is fixed on the support by means of anadhesive substance or by means of a surface treatment favouring atomicbonds.

In particular, this method according to the invention may be applied toobtain a thin film of ferroelectric material using a substrate offerroelectric material, and to fix it on a support.

Advantageously, the support being of semiconducting material, at leastone electronic control circuit is elaborated on one face of this supportand the thin film of ferroelectric material is fixed on the support soas to serve as a dielectric for a memory capacitor controlled by saidelectronic control circuit to thus make up a memory point.

The electronic control circuit is preferably of MOS transistor type.

The method according to the invention may also be applied to obtain athin sapphire film on a support, a thin corrosion-resisting metal filmon a support or a thin film of magnetic material on a support.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood and further advantages andparticularities will become apparent upon reading the description whichfollows, provided as a non-limiting example, made with reference to theattached drawings in which:

FIG. 1 is a partial cross-sectional view of an integrated circuitrealized on one face of a semiconductor substrate,

FIG. 2 illustrates the ion implantation step performed through one faceof a substrate of ferroelectric material, according to the presentinvention,

FIG. 3 illustrates the fixing step according to the present invention,which consists of making the face of the semiconductor substrate wherethe integrated circuit has been realized adhere to the face of thesubstrate of ferroelectric material which has been bombarded with ions,

FIG. 4 illustrates the step of the method according to the inventionleading to the separation of the thin film from the rest of thesubstrate of ferroelectric material,

FIG. 5 is a partial sectional view of a ferroelectric capacitor memorypoint realized according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method according to the invention is applicable to the followingcrystalline or noncrystalline solid materials:

insulating or dielectric materials,

conducting materials,

unorganized semiconducting materials,

semi-insulating materials, mainly those whose resistivity at ambienttemperature is superior to approximately 10⁷ Ω.cm,

monocrystalline metals and superconductors in general.

It is thus possible, according to the invention, to achieve thin filmsof monocrystalline quartz using solid monocrystalline quartz. It is alsopossible to obtain thin films of magnetic, piezoelectric, ferroelectric,pyroelectric materials and materials having non-linear opticalproperties or electro-optic, acousto-optical effects.

A particular example will now be described: the achievement offerroelectric capacitor memories on an integrated circuit.

The electronic circuit shown in the sectional view of FIG. 1 wasachieved using current micro-electronic techniques. The so-called “plug”technique, the mechanical-chemical oxide planarization technique and theso-called “damask” technique making it possible to achieve connectionsimbedded in an oxide but flush with the surface of the oxide wereimplemented.

The circuit was elaborated on a face 2 of a substrate 1 of type Psilicon. Implanted regions were realized on the face 2, only implantedregions 31, 32 and 33 of type N⁺being shown in this figure, and thefield oxide was increased to obtain insulation areas 41 and 42 to theleft of implanted region 31 and to the right of implanted region 33.Implanted regions 31 and 33 are intended to make up the drains of twoMOS type transistors, implanted region 32 making up their common source.On the face 2, rows of polycrystalline silicon words 51 and 52 have beendeposited and thin oxide layers 61 and 62 have been interposed. The rowsof words 51 and 52 have been covered with layers of insulating material65 and 66. This insulating material also covers areas 41 and 42 aslayers 63 and 64. A row of aluminium bits 8 ensures the electricalcontact with the source 32. An oxide layer 7 has been deposited to coverall the elements previously described. Flush platinum electrodes 91 and92 provided with TiN barrier sub-layers are deposited in the oxide layer7. The electrodes 91 and 92 are connected by “plugs” 11 and 12 totransistor drains 31 and 32. They are imbedded, the circuit then havinga flat external face 15.

The achievement of a thin film of ferroelectric material according tothe method of the present invention will now be described, this thinfilm being intended to form the capacitor dielectric.

FIG. 2 shows a side view of a substrate 100 of ferroelectric material,for example of PbZrTiO₃ (PZT). The flat face 101 of the substrate 100 isbombarded with ions, for example hydrogen ions at 200 keV and in a doseequal to 10 ¹⁷ cm⁻². The ion bombardment is indicated by arrows in FIG.2. The implanted ions induce the formation of microcavities distributedin a layer 102 adjoining a plane parallel to the flat face 101, thisplane being located at a distance from the flat face 101 correspondingto the average ion penetration depth. The layer 102 of implantedmaterial has a very small thickness, in order of a few tens of nm, forexample 50 to 100 nm. It separates the substrate 100 into two regions: afirst region 103 located on the flat face 101 side and intended to formthe thin film, and a second region 104 forming the rest of thesubstrate. The thickness of the region 103 is of approximately 800 nm.The layer 102 consists of a layer of microcavities.

The flat face 101 of the substrate of ferroelectric material 100 and theflat face 15 of the electronic circuit achieved on the semiconductorsubstrate 1 are treated, for example by a chemical process, so as tomake them capable of adhering to one another when merely placed incontact. FIG. 3 shows the two substrates 100 and 1 associated, the flatface 15 of the semiconductor substrate 1 adhering to the flat face 101of the substrate 100 of ferroelectric material.

The assembly is then heat treated at approximately 500° C., whichresults in inducing a separation of the two regions 103 and 104 of thesubstrate 100 of ferroelectric material at layer 102, as shown in FIG.4. A semiconductor substrate provided with an electronic circuit with athin film of ferroelectric material fixed to it is thus obtained.

The external face 105 of the thin film 103 may be finely polished.

The device shown in FIG. 5 is obtained, in which a two-capacitor memorypoint is formed by depositing a common electrode 16 on the flat face 105of the thin film 103.

A final packaging, may be added to protect the whole circuit.

Such a thin ferroelectric film may also be used to make up a layer offerroelectric material deposited directly on the silicon to achieve MOStransistors in which the control gate is replaced with thisferroelectric layer whose polarization state determines the off oron-state of the transistor.

In particular, the application of the method according to the inventionto dielectric materials makes it possible to achieve sapphirewear-resisting layers (β-alumina) on glass or silica supports. Such athin alumina layer makes it possible to protect the glass or silicaserving as a support, for example for optical components, from wear andscratches. An implantation of hydrogen ions at approximately 8×10¹⁶atoms/cm² and 110 keV makes possible a thin sapphire layer approximately1 μm thick. This small thickness is compatible with a subsequent shapingof the glass or silica serving as the support so as to achieve lensesfor example.

The method according to the invention is also applicable to metalmaterials. It makes it possible to achieve anticorrosive layers anddiffusion barriers. The possibility of achieving monocrystalline metallayers instead of polycrystalline layers provides a significantadvantage in terms of efficiency as a diffusion barrier against chemicalaggressions and corrosion in particular. Indeed, the existence ofimportant diffusion phenomena at the grain joints in polycrystallinematerials limits the efficiency of the thin layers achieved in thesematerials. As an example, consider depositing a thin film ofmonocrystalline niobium 500 nm thick on a steel substrate to achieveobjects intended to resist high temperatures in corrosive mediums. Toobtain this thin film, an implantation of H⁺ions at approximately 2×10¹⁷atoms/cm² and 200 keV may be implemented.

Another example of an application concerns the achievement of memoriesusing magnetic domains (bubbles) and magnetic domain walls (Block walls)to store the information. For this purpose, a solid substrate ofnon-magnetic garnet may be used on which a layer of ferrimagnetic garnetis developed by epitaxial growth. The method according to the inventionmakes it possible to add a thin layer of ferrimagnetic garnet materialon a silicon substrate serving as a support and comprising integratedcircuits. These integrated circuits combine electronic, logical, andanalog devices, and integrated microwindings suited to generatelocalized magnetic fields to drive, displace and detect the magneticdomains or domain walls in the thin layer of ferrimagnetic garnet.

What is claimed as new and is desired to be secured by Letters Patent ofthe United States is:
 1. A method for making a thin film of solidmaterial, comprising the steps of: bombarding one face of a substrate ofthe solid material with at least one of rare gas ions and hydrogen gasions so as to create a layer of microcavities separating the substrateinto two regions at a depth neighboring the average ion penetrationdepth; and heating the layer of microcavities to a temperaturesufficient to bring about a separation between the two regions of thesubstrate, wherein said solid material comprises one of a dielectricmaterial, a conducting material, a semi-insulating material, and anunorganized semiconducting material.
 2. A method according to claim 1,further comprising fixing said face of the substrate on a supportbetween the bombarding step and the heating step.
 3. A method accordingto claim 2, wherein said face of the substrate is fixed on the supportby an adhesive substance.
 4. A method according to claim 2, wherein saidface of the substrate is fixed on the support by a treatment favoringatomic bonds.
 5. A method according to claim 2, wherein: said solidmaterial comprises ferroelectric material; the support comprisessemiconducting material and includes at least one electronic controlcircuit elaborated on one face of the support; and the thin film offerroelectric material is fixed on the support so as to serve as adielectric for a memory capacitor controlled by said electronic controlcircuit to thus make up a memory point.
 6. A method according to claim5, wherein the electronic control circuit comprises a MOS transistortype circuit.
 7. A thin sapphire film on a support made by the methodaccording to claim
 2. 8. A thin corrosion-resisting metal film on asupport made by the method according to claim
 2. 9. A thin film ofmagnetic material on a support made by the method according to claim 2.10. A thin sapphire film on a support made by the method according toclaim 2, wherein said face of the substrate is fixed on the support byat least one of an adhesive substance and a treatment favoring atomicbonds.
 11. A thin corrosion-resisting metal film on a support made bythe method according to claim 2, wherein said face of the substrate isfixed on the support by at least one of an adhesive substance and atreatment favoring atomic bonds.
 12. A thin film of magnetic material ona support made by the method according to claim 2, wherein said face ofthe substrate is fixed on the support by at least one of an adhesivesubstance and a treatment favoring atomic bonds.
 13. A method accordingto claim 1, wherein said solid material comprises ferroelectricmaterial.
 14. A thin sapphire film on a support made by the methodaccording to claim
 1. 15. A thin corrosion-resisting metal film on asupport made by the method according to claim
 1. 16. A thin film ofmagnetic material on a support made by the method according to claim 1.17. A method according to claim 1, wherein the two regions of thesubstrate are separated naturally.
 18. A method according to claim 1,wherein the two regions of the substrate are separated by applyingstress.